14C emits negative beta particles with 165 keV maximal energy. Its half-life is 5,730 years. It is produced by the 14N(n, p)14C nuclear reaction. B-, Be-, or Al — nitrides are used as the target, and it is oxidized after irradiation, e. g., by hydrogen peroxide. By this method, 14CO2 can be obtained, and it then can be dissolved in NaOH and precipitated with Ba(OH)2 as Ba14CO3. This is the basic compound, which is mostly used in organic syntheses. The procedure is shown in Figure 8.10. For organic syntheses, Ba14CO3 is transformed in several different ways:

1. It can be dissolved in hydrochloric acid. In this process, 14CO2 is formed. From 14CO2, the following compounds can be produced:

• Carboxylic acids (e. g., acetic acid) can be prepared in the Grignard reaction:

14CO2 1 CH3MgBr 12H2O! CH314COOH 1 Mg(OH)2 1 HBr (8.22)

The acetic acid is specifically labeled on the carboxylic carbon atom.

• 14CO2 can be reduced with LiAlH4 to methanol.

• Methanol can be converted to methyl iodide, formaldehyde, or methyl cyanide.

2. By heating Ba14CO3, different compounds are obtained, dependent on the conditions and reagents:

• With metallic potassium in molten NH4Cl, K14CN is produced.

• With metallic barium, it gives universally labeled acetylene:

Ba14CO3 1 Ba ! Ba14C2 1 H2O! 14C2H2 (8.23)

• Acetylene can be used in many syntheses, e. g., universally 14C-labeled benzene. By addition of water, acetaldehyde is obtained. In the reaction of labeled acetylene with formaldehyde, heterocyclic compounds with oxygen as a heteroatom can be produced.

Ba(OH)2

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Ba14CO3

Filtration, washing, drying

Figure 8.10 The separation of C-14 from Be3N2 and preparation of Ba14CO3.

• By heating Ba14CO3 in dry NH3, barium cyanamide is produced, from which urea, thiourea, and guanidine can be synthesized.

3. By the reduction of NaH14CO3 or KH14CO3 by H2, formic acid is produced, using Pd as a catalyst.

4. A special problem of organic chemistry is the preparation of labeled aromatic compounds. As seen previously, universally 14C-labeled benzene can be produced from 14C2H2.

As mentioned in Section 8.4, biological syntheses are also possible:

• Clostridium aceticum is produced from CO2 to acetic acid.

• Chlorella vulgaris is produced from CO2 to amino acid in an L-configuration.

• Canna indica is produced from CO2 to carbohydrate in a D-configuration.

• Pigeon produces from formiate to acetic acid.

• Rat produces from acetate to cholesterol.